1. Field of the Invention
The present invention relates to an image processing system suitable for printing or displaying images in multiple tonal ranges; and more particularly to improvements in techniques for expressing multiple tonal ranges with a minimum amount of data. The present invention is therefore suitable for an image output apparatus, such as a printer or a cathode ray tube display unit, which is capable of displaying luminosity or brightness.
2. Discussion of the Related Art
A dot matrix printer forms an image by printing the pixels of an image in either of the binary color values black and white. When such a dot matrix printer forms a multi-tonal range image, it uses a plural number of pixels as one unit of a tonal range expressing area. This tonal range expressing area is processed using a dither method or a screen method. In such methods, luminosity in multi-tonal ranges results from the number of black pixels in a screen area. In these methods, the number of tonal ranges of luminosity equals the number of pixels forming a tonal range expressing area plus 1.
In recent years, laser printer technology has improved remarkably. Laser printers can produce variations in the density of a single pixel by controlling the energy of a laser beam applied to the pixel. Laser printers using this type of density variation are available and can produce a large number of tonal variations. Laser printers of this type use a pixel information including several bits of tonal range data and control the laser energy by generating a time corresponding to the tonal range data for each pixel by using a digital-to-analog (D/A) converter.
However, these laser printers have a a limit to the number of tonal ranges available. This limit is approximately five for a pixel size of approximately 40 .mu.m.sup.2.
Using a tonal range expressing area composed of a plural number of pixels according to the dot matrix techniques coupled with the method employed by the laser printer technique an increased number of tonal ranges can be expressed. For example, a tonal range expressing area comprising 16 pixels in a four by four arrangement, the number of tonal ranges which can be expressed will be approximately 65. The amount of data for expressing four tonal ranges is originally allocated to each pixel, however, so that the amount of data necessary for expressing one tonal range is not less than what is expressed by the following equation: EQU (log .sub.2 5).times.16=37.2 bits (1)
Accordingly, the amount of the data necessary for expressing one tonal range will be more than six times as much as the amount necessary for expressing 65 tonal ranges in a gray scale image, as indicated by the following equation: EQU log.sub.2 65=6.03 bits (2)
Furthermore, most images are ordinary documents and drawings which have only the binary values white and black. In these images, the amount of data per 16 pixels for the expression of such a binary value image is expressed by the following equation: EQU (log.sub.2 2).times.16=16 bits (3)
This amount is two to three times or less than the amount of data required for one tonal range expressing area as mentioned above.
Therefore, the conventional multi-tonal range image processing system using a tonal range expressing area requires a memory capacity several times greater and an image transfer speed greater than a gray scale image processing system or binary value image processing system, resulting in a more expensive conventional image processing system.
Thus, an image output device which is capable of expressing images in multi-tonal ranges using a minimum of data without requiring any increased memory capacity is needed.